Electrical machines (motors, generators) have, in the multiplicity of their longitudinal grooves in the stator laminate stacks, special types of coil windings or conductor bars, generally made from copper or another material of high conductivity.
In the case of an electric motor, by supplying current in a time-selective manner, a magnetic field propagating in all directions is generated driving the freely rotating rotor suspended in the stator cavity with the rotor reacting to the induced magnetic field in the form of forced rotation, owing for example to a multiplicity of applied permanent magnets and thus, converting electrical energy into kinetic energy. In electrical terms, a stator laminate stack is at ground potential, while coils are at high kilovolt potential. The coils fitted into the stator grooves must therefore be electrically insulated with respect to ground potential. For this purpose, each and every coil is insulated with, for example, a specific tape, mica tape for example, repeatedly and with defined overlap.
Mica is used preferentially because, being a particulate, more particularly lamellar, inorganic barrier material, it is capable of retarding electrical erosion under electrical partial discharges effectively and for a long time, as for example over the entire lifetime of the machine, and has good chemical and thermal stability. Mica tapes consist of mica paper and one or more carriers, for example fabrics, film(s), which are joined to one another via a tape adhesive. Mica tapes are necessary since mica paper alone does not have the mechanical strength needed for an insulation process. Depending on application, additives may be added to the tape adhesive, examples being curing catalysts, which have an initiating effect on the thermal curing of an externally applied impregnating agent: after the mica tape-insulated coils have been fitted into the stator laminate stacks and connected electrically, for avoidance of partial discharges during later operation, the air in the cavities of the windings and especially in the groove gaps of the stator laminate stack is eliminated. Because this distance from current-carrying insulated coil to the laminate stack is generally kept as small as possible, field strengths of several kV/ram there are not unusual. There is corresponding stress on the insulation material.
Impregnating agents according to the prior art that have proven suitable for vacuum impregnation processes include thermally curable epoxy resin/anhydride mixtures.
They are used for impregnation of the stators of the electrical machines, composed of the individual parts thereof, with the fitted and mica tape-insulated coils, or for individual impregnation of coils or conductor bar.
During a specific vacuum impregnation process, the VPI (vacuum pressure impregnation) process, these stators or coils have to date been wholly flooded with a mobile epoxy resin/phthalic anhydride formulation in a vacuum chamber and then impregnated under pressure. The final cure takes place in general under atmospheric pressure in an industrial oven. The function of the curing catalyst here is to gel the mobile impregnating agent, commonly composed of epoxy resin and phthalic anhydride, within a certain time at a predetermined temperature. The industrial standard impregnating agent for this purpose to date is a mixture of distilled bisphenol A diglycidyl ether and methylhexahydrophthalic anhydride. This mixture is sufficiently mobile to ensure the complete impregnation of the tape insulation on the one hand and, in the absence of curing catalysts, a sufficient storage stability, on the other. The curing catalyst is generally present at least also in the solid insulating material, e.g., mica tape. This mica tape is held together via the tape adhesive, and so it is essential that the tape adhesive and the curing catalyst are inert to one another.
More particularly, it is advantageous if all three components, i.e., tape adhesive, curing catalyst, and charged impregnating agent, do not react until the moment they encounter one another during the VPI process. In this way, the best possible crosslinking and attachment, compatibility and non-cavitation are achieved for the insulation, this leading in turn to an optimized lifetime of the “main insulation” of the electrical machine, that comes about thereafter in the course of curing.
Owing to toxicological concerns about the unrestricted use of phthalic anhydrides, impregnating agents used in the future will be phthalic anhydride-free or completely anhydride-free, epoxide-based impregnating agents, polymerized using curing catalysts.
The new curing catalysts will be matched to the anhydride-free impregnating agents. Anhydride-free impregnating agents are known from the prior applications DE 102014219844.5, DE 102014221715.6, DE 102015205328.8, DE 102015202053.3, DE 102015208527.9, DE 102015204885.3, the disclosure content of which is hereby incorporated into the present description, will increasingly be used. Those applications describe how the curing catalysts employed to date do not produce sufficient catalysis in the case of the epoxy resin-based, anhydride-free impregnating resins which will be used in future, and so the resultant formed materials are either too soft, hence exhibiting inadequate mechanical, thermomechanical and/or thermal property values, or do not form shaped bodies at all, since the conventional catalysts simply do not cure the new impregnating resins.
Hence it has been found that nitrogen heterocycles, such as imidazoles, constitute effective gelling and/or curing catalysts for acid anhydride-free epoxy resins based on bisphenol A and/or on bisphenol F diglycidyl ether.
Thus, for example, an acid anhydride-free bisphenol F diglycidyl ether, gelled with 3 wt % of a prior-art curing catalyst, such as an N-alkyl-substituted piperazine derivative, and subjected to anionic polymerization curing at 145° C. for 10 hours, produces only a glass transition of around 90° C., whereas the standard anhydride-containing epoxy resin and curing catalyst under identical curing conditions develops a glass transition of around 160° C.
If, conversely, 2 wt % of 1,2-dimethylimidazole is used as gelling and curing catalyst for an anhydride-free impregnating resin based on epoxy resin, such as bisphenol F diglycidyl ether, for example, then the glass transition that comes about is up to 150° C.
A disadvantage of the imidazoles, however, is that the vapor pressures of the imidazoles at elevated temperatures are relatively high, and so there is a risk of partial expulsion from the mica tape binder during long-lasting evacuation phases at elevated temperatures, of the kind employed in the production of electrical machines prior to the vacuum impregnation of the stators, for the purpose of preliminary drying, for instance.
This may also be accompanied by the disadvantageous phenomenon of entrainment of the volatile imidazoles into the VPI resin reservoir during the impregnating phase, something which in turn shortens the storage stability of the impregnating resin itself.